Epsin and AP180 are essential components of the endocytotic machinery,which controls internalization of protein receptors and othermacromolecules at the cell surface. Epsin and AP180 are recruited to theplasma membrane by their structurally and functionally related N-terminalENTH and ANTH domains that specifically recognize PtdIns(4,5)P2. Here, weshow that membrane anchoring of the ENTH and ANTH domains is regulated bythe acidic environment. Lowering the pH enhances PtdIns(4,5)P2 affinity ofthe ENTH and ANTH domains reinforcing their association with lipidvesicles and monolayers. The pH dependency is due to the conservedhistidine residues of the ENTH and ANTH domains, protonation of which isnecessary for the strong PtdIns(4,5)P2 recognition, as revealed byliposome binding, surface plasmon resonance, NMR, monolayer surfacetension and mutagenesis experiments. The pH sensitivity of the ENTH andANTH domains is reminiscent to the pH dependency of the FYVE domainsuggesting a common regulatory mechanism of membrane anchoring by a subsetof the PI-binding domains.

The epsin N-terminal homology (ENTH) domain is a conserved protein modulepresent in cytosolic proteins which are required in clathrin-mediatedvesicle budding processes. A highly similar, yet unique module is theAP180 N-terminal homology (ANTH) domain, which is present in a set ofproteins that also support clathrin-dependent endocytosis. Both ENTH andANTH (E/ANTH) domains bind to phospholipids and proteins, in order tosupport the nucleation of clathrin coats on the plasma membrane or thetrans-Golgi-network membrane. Therefore, E/ANTH proteins might beconsidered as universal tethering components of the clathrin-mediatedvesicle budding machinery. Since the E/ANTH protein family appears to becrucial in the first steps of clathrin-coated vesicle budding, weperformed data base searches of the Arabidopsis thaliana genome. Sequenceanalysis revealed three proteins containing the ENTH signature motif andeight proteins containing the ANTH signature motif. Another six proteinswere found that do not contain either motif but seem to have the samedomain structure and might therefore be seen as VHS-domain-containingplant proteins. Functional analysis of plant E/ANTH proteins are ratherscarce, since only one ANTH homolog from A. thaliana, At-AP180, has beencharacterized so far. At-AP180 displays conserved functions as a clathrinassembly protein and as an alpha-adaptin binding partner, and in additionshows features at the molecular level that seem to be plant-specific.

The interaction of the human GGA1 GAT domain with rabaptin-5 is mediatedby residues on its three-helix bundle.

Biochemistry. 2003; 42: 13901-8

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GGA proteins regulate clathrin-coated vesicle trafficking by interactingwith multiple proteins during vesicle assembly. As part of this process,the GAT domain of GGA is known to interact with both ARF and Rabaptin-5.Particularly, the GAT domains of GGA1 and -2, but not of GGA3,specifically bind with a coiled-coil region of Rabaptin-5. Rabaptin-5interacts with Rab5 and is an essential component of the fusion machineryfor targeting endocytic vesicles to early endosomes. The recentlydetermined crystal structure of the GGA1 GAT domain has provided insightsinto its interactions with partner proteins. Here, we describe mutagenesisstudies on the GAT-Rabaptin-5 interaction. The results demonstrate that ahydrophobic surface patch on the C-terminal three-helix bundle motif ofthe GAT domain is directly involved in Rabaptin-5 binding. A GGA3-likemutation, N284S, in this Rabaptin-5 binding patch of GGA1 led to a reducedlevel of Rabaptin-5 binding. Furthermore, a reversed mutation, S293N, inGGA3 partially establishes Rabaptin-5 binding ability in its GAT domain.These results provide a structural explanation for the binding affinitydifference among GGA proteins. The current results also suggest that thebinding of GAT to Rabaptin-5 is independent of its interaction with ARF.

The high energy potential and rapid turnover of the recently discoveredinositol pyrophosphates, such as diphosphoinositol-pentakisphosphate andbis-diphosphoinositol-tetrakisphosphate, suggest a dynamic cellular role,but no specific functions have yet been established. Using several yeastmutants with defects in inositol phosphate metabolism, we identifydramatic membrane defects selectively associated with deficient formationof inositol pyrophosphates. We show that this phenotype reflects specificabnormalities in endocytic pathways and not other components of membranetrafficking. Thus, inositol pyrophosphates are major regulators ofendocytosis.

Simultaneous binding of PtdIns(4,5)P2 and clathrin by AP180 in thenucleation of clathrin lattices on membranes.

Science. 2001; 291: 1051-5

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Adaptor protein 180 (AP180) and its homolog, clathrin assembly lymphoidmyeloid leukemia protein (CALM), are closely related proteins that playimportant roles in clathrin-mediated endocytosis. Here, we present thestructure of the NH2-terminal domain of CALM bound tophosphatidylinositol-4,5- bisphosphate [PtdIns(4,5)P2] via a lysine-richmotif. This motif is found in other proteins predicted to have domains ofsimilar structure (for example, Huntingtin interacting protein 1). Thestructure is in part similar to the epsin NH2-terminal (ENTH) domain, butepsin lacks the PtdIns(4,5)P2-binding site. Because AP180 could bind toPtdIns(4,5)P2 and clathrin simultaneously, it may serve to tether clathrinto the membrane. This was shown by using purified components and a buddingassay on preformed lipid monolayers. In the presence of AP180, clathrinlattices formed on the monolayer. When AP2 was also present, coated pitswere formed.

Structure of the VHS domain of human Tom1 (target of myb 1): insights intointeractions with proteins and membranes.

Biochemistry. 2000; 39: 11282-90

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VHS domains are found at the N-termini of select proteins involved inintracellular membrane trafficking. We have determined the crystalstructure of the VHS domain of the human Tom1 (target of myb 1) protein to1.5 A resolution. The domain consists of eight helices arranged in asuperhelix. The surface of the domain has two main features: (1) a basicpatch on one side due to several conserved positively charged residues onhelix 3 and (2) a negatively charged ridge on the opposite side, formed byresidues on helix 2. We compare our structure to the recently obtainedstructure of tandem VHS-FYVE domains from Hrs [Mao, Y., Nickitenko, A.,Duan, X., Lloyd, T. E., Wu, M. N., Bellen, H., and Quiocho, F. A. (2000)Cell 100, 447-456]. Key features of the interaction surface between theFYVE and VHS domains of Hrs, involving helices 2 and 4 of the VHS domain,are conserved in the VHS domain of Tom1, even though Tom1 does not have aFYVE domain. We also compare the structures of the VHS domains of Tom1 andHrs to the recently obtained structure of the ENTH domain of epsin-1[Hyman, J., Chen, H., Di Fiore, P. P., De Camilli, P., and Brunger, A. T.(2000) J. Cell Biol. 149, 537-546]. Comparison of the two VHS domains andthe ENTH domain reveals a conserved surface, composed of helices 2 and 4,that is utilized for protein-protein interactions. In addition, VHSdomain-containing proteins are often localized to membranes. We suggestthat the conserved positively charged surface of helix 3 in VHS and ENTHdomains plays a role in membrane binding.

Epsin binds to clathrin by associating directly with the clathrin-terminaldomain. Evidence for cooperative binding through two discrete sites.

J Biol Chem. 2000; 275: 6479-89

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Epsin is a recently identified protein that appears to play an importantrole in clathrin-mediated endocytosis. The central region of epsin 1, theso-called DPW domain, binds to the heterotetrameric AP-2 adaptor complexby associating directly with the globular appendage of the alpha subunit.We have found that this central portion of epsin 1 also associates withclathrin. The interaction with clathrin is direct and not mediated byepsin-bound AP-2. Alanine scanning mutagenesis shows that clathrin bindingdepends on the sequence (257)LMDLADV located within the epsin 1 DPWdomain. This sequence, related to the known clathrin-binding sequences inthe adaptor beta subunits, amphiphysin, and beta-arrestin, facilitates theassociation of epsin 1 with the terminal domain of the clathrin heavychain. Unexpectedly, inhibiting the binding of AP-2 to the GST-epsin DPWfusion protein by progressively deleting DPW triplets but leaving theLMDLADV sequence intact, diminishes the association of clathrin inparallel with AP-2. Because the beta subunit of the AP-2 complex alsocontains a clathrin-binding site, optimal association with solubleclathrin appears to depend on the presence of at least two distinctclathrin-binding sites, and we show that a second clathrin-bindingsequence (480)LVDLD, located within the carboxyl-terminal segment of epsin1, also interacts with clathrin directly. The LMDLADV and LVDLD sequencesact cooperatively in clathrin recruitment assays, suggesting that theybind to different sites on the clathrin-terminal domain. The evolutionaryconservation of similar clathrin-binding sequences in several metazoanepsin-like molecules suggests that the ability to establish multipleprotein-protein contacts within a developing clathrin-coated bud is animportant aspect of epsin function.